Surface-based meteorological stations may be used in connection with monitoring a variety of environmental conditions. In connection with surface-based meteorological stations, it is often desirable to measure the speed and direction of the wind. However, surface-based meteorological stations are often unable to take accurate measurements using mechanical anemometers. In particular, unless an anemometer is located in a flat, open area where the wind can freely pass over the surface, it is not possible to obtain accurate wind data. As a result, few backyard or other small scale weather stations can accurately measure the wind due to the effects of surrounding houses and trees.
Electronic devices for remotely measuring atmospheric conditions using radio and/or audio frequencies are available. Such devices have been designed to take measurements at large distances (e.g., 200 meters or greater) above the surface. As a result, relatively long wavelength, high powered radars and/or loudspeakers arc required. Because of the large altitudes from which such devices are designed to obtain data, large antennas or arrays of antennas must be used in order to transmit the radio frequency signal in a narrow beam. Similarly, large loudspeakers or arrays of loudspeakers are often required. Accordingly, remote sensing systems arc typically large, expensive, and consume large amounts of power.
Sound detection and ranging (SODAR) systems use sound waves for remotely measuring the wind. In a SODAR system, acoustic pulses are sent upward and the Doppler shift and acoustic energy reflected back to the device by the atmosphere are measured. In a typical implementation, at least three acoustic beam paths are used, although the accuracy of data can be increased by using a larger number of beam paths. SODAR systems are typically used to measure winds at altitudes of up to hundreds of meters. The altitude at which reflection occurs is determined by the time required for the reflected energy to be received. SODAR systems rely on small scale turbulence to reflect the acoustic energy back to the device. Because naturally occurring turbulence is required to reflect the sound energy, SODAR systems can be limited in the altitudes from which data is obtained.
Wind profiling radar systems have also been utilized to remotely sense the speed and direction of the wind. A wind profiling radar depends on refractive irregularities provided by eddies in the atmosphere to scatter the radar beam, thereby creating a return signal. By measuring the Doppler shift observed in connection with returned signals, the velocity of the wind can be determined. In a typical wind profiler radar, measurements are made in five directions using radar beams having a beamwidth of less than 10 degrees. Wind profiler radars are capable of obtaining wind data at heights of many kilometers. However, in order to obtain such range, wavelengths of about 33-75 cm, and correspondingly large antennas and power supplies, are required. In addition, as with SODAR systems, wind profiling radar can be limited in the altitudes from which data is obtained.
Systems combining the use of acoustic and radio frequency signals, known as radio-acoustic sounding systems (RASS), have been used to obtain vertical profiles of temperature over the location of the system. To obtain a temperature profile, a range of audio frequencies with wavelengths about one-half that of the radar are transmitted into the atmosphere. The exact wavelength of the sound frequency is a function of the air temperature that the sound wave is passing through. When the wavelength of the sound is exactly one-half the radar wavelength, Bragg scattering of the transmitted radar pulse will occur from that segment of the sound pulse. Therefore, by transmitting a range of audio frequencies and observing the frequencies at which a return is generated to the radar transceiver, the temperature of the atmosphere at selected altitudes can be obtained. Such systems are effective at providing temperature profiles up to altitudes of several kilometers. In order to achieve accurate measurements at such altitudes, pencil beam radars (e.g., having a beamwidth of less than 10°) are used. Radio-acoustic sounding systems are often combined with wind profiler radar. Because of the long ranges at which such devices operate, large loud speakers, and large radar antennas capable of producing very narrow beams are required.
Systems that use a combination of audio frequencies and radio frequencies to determine wind speed and direction have also been developed. In such systems, a sound pulse, which will be blown from its initial trajectory by wind in the atmosphere, and an associated directional radar arc aimed so that they intersect at a location in the atmosphere from which data is to be obtained. The sound pulse has a wavelength that is one-half the wavelength of the radio frequency signal, so that the sound pulse reflects the radio frequency signal. The Doppler frequency of the returned signal is used to compute wind velocity, and directional measurements can be made based on the direction in which the radar is aimed. Such a system is described in U.S. Pat. No. 4,761,650 to Masuda et al. (“Masuda”), the disclosure of which is hereby incorporated herein by reference in its entirety. In an exemplary embodiment, Masuda describes a system that generates sound waves having a frequency of 70-120 Hz at a power of 200 W, and that includes a radar having a frequency of 46.5 MHz, a power of 1 MW, and a beamwidth of 3-6°. Because such systems require large acoustic and radar frequency transducers, and because they require large amounts of power, they are not suitable for small-scale installations.
Another system using a combination of audio and radio frequencies to measure wind is described in U.S. Pat. No. 4,351,188 to Fukushima et al. (“Fukushima”), the disclosure of which is hereby incorporated herein by reference in its entirety. The Fukushima reference describes measuring wind by directing a sound pulse vertically, and using a multiplicity of antennas to detect the point to which continuously generated radio frequency waves are reflected by the sound pulse. Accordingly, the system of Fukushima requires a lattice of receiving antennas laid out over a large area. In an exemplary embodiment, Fukushima describes a system using acoustic signals having frequencies from 680 to 6800 Hz and a sound pressure level of 130 to 150 dB, radio frequency signals having a frequency of from 300 to 3000 MHz, and a wavelength of from 0.1 to 1.0 m transmitted from aperture antennas having a diameter of 0.2 to 2 m. For a system capable of detecting wind at altitudes of up to 1000 m and at speeds of up to 5 m/sec, Fukushima describes a lattice of receiving antennas distributed over a square area measuring 30 m on each side.
Systems for remotely measuring precipitation parameters are also available. However, the accurate detection of precipitation requires much shorter radio frequency wavelengths than those typically used by radar systems for measuring other atmospheric conditions. In particular, devices for measuring wind or temperature have used relatively long wavelengths to enable sensing at high altitudes, and are therefore ineffective at measuring precipitation. Accordingly, a device for measuring precipitation must typically be provided separately from devices used to detect the wind or the air temperature remotely.
Accordingly, it would be desirable to provide a system for remotely sensing the wind that is relatively compact and inexpensive. In addition, it would be desirable to provide such a system that was capable of reliable operation in a wide range of weather conditions. Furthermore, it would be desirable to provide a system capable of remotely sensing the wind that was also capable of measuring precipitation without requiring additional hardware.